Before the materials in rubbish or waste can be supplied for reuse these materials must be separated. Inter alia, the specific gravity or density of the materials provides a criterion for the separation. In practice many processes are known for the separation, e.g. air classification, winnowing, flotation, etc. To use these processes effectively one necessary condition is that there is only a relatively small difference between the smallest and largest particles. When this condition is met, the mass can be separated into specific materials of the same density. According to the kind of further processing, e.g. particularly in processing to building materials, the above-mentioned separation criterion suffices for an effective fractionation. Should separation of individual materials be necessary a further separation according to criteria specific to the materials must be performed.
In one known process the waste is comminuted, dried and then separated into fractions by one of the above-mentioned methods. The comminution is to ensure substantially uniform particle size, and the drying is to prevent the individual particles from sticking together.
Since it is hardly possible to start with waste that has a substantially uniform particle size from the beginning, the known separation process involves a substantial expenditure of work caused by the comminution, by which the processing time is considerably increased.
Specific devices such as classification devices, flotation devices and the like are used for performing the above-mentioned separation process. Not only are such devices expensive, but they also need to be constructed on a large scale in order to produce the effects needed to bring about the separation.